1. Field of the Invention
The present invention relates to a quality management system for a semiconductor device, and more particularly to a quality management system used in an in-line inspection.
2. Description of the Background Art
In order to increase and stabilize a yield of a semiconductor device, an inspection step is executed in a manufacturing line (in-line inspection) to observe the number of nonconformities (hereinafter referred to as "defects") between a semiconductor device under manufacture and that on design, and if the number of defects exceeds a specified upper limit, the defects are probed and what is the defect source is estimated, to get rid of the defect source.
FIG. 7 is a conceptional diagram showing a manufacturing line of a semiconductor device and an in-line inspection executed therein. A process for manufacturing a semiconductor device needs over two hundred steps only for dealing a wafer, and it sometimes takes two months or more from start to finish. In such a case, with an inspection (quality evaluation) after completion, all products through the process that is the defect source during the past two months, at the worst, may have defects found in the inspection, if any, resulting in a serious damage. In order to suppress the damage to the least, the manufacturing process is divided into blocks, each consisting of associated steps, and the inspection (quality evaluation) is made on a block-by-block basis as shown in FIG. 7 so that only two-or-three-days' damage is given if any defect is found. In FIG. 7, the manufacturing process is divided into blocks BLI to BLn and in-line inspection steps IE1 to IEn are provided at the respective last stages of the blocks.
An outline of a prior-art in-line inspection will be discussed with reference to FIG. 8. First, some of wafers through the last step of a block are taken into an inspection device 1. As the inspection device 1 used may be an optical system utilizing intensity of scattered light for detection of defects, a mechanical system for mechanical detection of defects and the like. The inspection device of both types obtains measured data D1 such as positional coordinates and size of the defects to be given to a quality management system S90 for performing a quality management based thereon.
The quality management system S90 has a measured-data judgment unit 4 for comparing the number of defects and the number of chips having a defect with predetermined values (upper control limit values). If the measured-data judgement unit 4 makes a judgment that the number of defects or the number of chips having a defect exceeds the respective upper limits of the predetermined values, the unit 4 gives a warning or an operation instruction CM1 to associated apparatus 3 such as a semiconductor manufacturing apparatus.
As mentioned earlier, however, since the in-line inspection is made on a block-by-block basis for the semiconductor device through a plurality of (twenty to thirty) manufacturing steps, it is impossible to estimate what step causes the defect from only the data given by the inspection device 1. For this reason, the wafer to be inspected is set in an observation device 2 to analyze an image of the defect in detail. The observation device 2 comprises a magnifying device such as an optical microscope and an electron microscope and magnifies a defective portion with the aid of the positional information of the defect given by the inspection device 1, for observation.
The observation device 2 observes size and shape of the defect and condition of the defect and its periphery to thereby estimate what apparatus and process for manufacturing the semiconductor device may cause the defect (i.e., defect source) and give a warning or an operation instruction CM2 to the associated apparatus 3 such as a semiconductor manufacturing apparatus, if necessary.
Since the quality management system S90 as mentioned above is adopted in the prior-art in-line inspection, it is impossible to surely make a judgment of abnormal condition (deterioration in product yield) only because the number of defects or the number of chips having a defect exceeds the predetermined value.
Specifically, though an in-line inspection can detect a defect on a block-by-block basis and a probe can estimate the defect source, it is impossible to make a decision on what effect the defect has in all the steps, and in other words, whether the defect causes deterioration in product yield or not. Having a grasp of all the steps and knowing how an unsolved defect affects a final product makes it possible to make such a decision.
An human operator makes a judgment, from practical experience, on whether there is a defect that would cause deterioration in product yield, by observing defects one by one based on finding of defects on a block-by-block basis, and makes a decision on whether the manufacturing line should be stopped.
Therefore, it disadvantageously takes much time and labor from finding of defects to recognition of occurrence of abnormal condition in the conventional management system.
Further, the judgment from practical experience of a human operator on an influence degree (fatal or killer rate of the defect) has an accuracy problem as well as time and labor problems.